Progressive overload is the principle of gradually increasing the demand on your muscles over weeks and months. In practice, it means you manage more reps with the same weight or the same reps with more weight.
Why it matters so much
Muscles grow when they have to do more than they are used to. Without that progress, there is no reason for new muscle tissue, even in a calorie surplus. Progressive overload is therefore the most important training factor for building muscle, and rising performance is also one of the best signs that a muscle is growing.
Double progression
A proven scheme is double progression. You set a rep range, for example 8 to 12. You start with a weight at which you hit the bottom of the range in all sets and add reps from session to session. Once you reach the top of the range in all sets, you increase the weight by the smallest sensible step, usually 2.5 kg on a barbell, and start again at the bottom.
Example: Bench press with 60 kg for 3 sets of 8. Over a few sessions you work up to 3 sets of 12. Then you move to 62.5 kg and start again at 8. These numbers are an example.
No log, no progression
Without a training log, you will hardly notice whether you are progressing. Write down weight, reps and ideally the RPE or RIR of every work set. Stick with the same exercises for a few months, or you lose the comparison.
In a cut, the goal changes: there it is more about holding your weights. If performance drops over several sessions, a lighter week (deload) is often smarter than stubbornly pushing on.